Achieving isotropic or uniform spatial and angular illumination on a specimen is an important goal in microscopy. Equipment limitations and illumination techniques are factors that affect uniformity of illumination. Illumination techniques affect the quality of microscopic research, but conventional techniques are also problematic.
Conventional illumination techniques use light sources such as incandescent lamps/filaments, light emitting diodes, bulbs, or others. These light sources frequently have errors in spatial alignment that exceed the dimensions of surfaces at which light ray emission or propagation occurs. Consequently, light sources cannot fill an aperture sufficiently, requiring alignment of active elements during and/or diffusion within fixed elements to correct these errors. Alignment of active elements can be very effective, but it requires a skilled technician. An operator of a microscope typically does not possess the skills or training for active element alignments after replacement of a source. Active alignment of illumination system components requires training, frequent replacement parts, and can incur significant time delay while adjusting or replacing illumination sources. Diffusion within fixed elements spreads an image of the source but greatly reduces the coupling efficiency. Accommodations for reduced coupling efficiency typically shorten the lifetime of a source. The variable magnitude of these accommodations for reduced coupling efficiency can create color variations in the illumination. Consistent illumination is difficult and requires skills not held by an operator of a microscope. A trained technician in optical assembly of illumination systems may have difficulty achieving the required uniformity in both space and angle.
Incandescent sources are selected based on life span of the illumination elements, dimensions of filament area and power, all of which can affect brightness, uniformity, and color temperature within the illumination field. Incandescent lamps for microscopy display lifetimes in the range of 200 to 2000 hours of use within their nominal operating conditions. Short-lifetime incandescent sources display brighter exitance (exiting power per area) at the filament than long-lifetime sources. Consequently, a short-lifetime bulb can accommodate a low coupling efficiency while a long-lifetime bulb cannot tolerate a drop in coupling efficiency. A short-lifetime bulb can accommodate the poor coupling efficiency of diffusion within fixed elements, but the spatial uniformity can display a deviation of 30 to 40% from its peak. The operating current of a short-lifetime bulb can be varied for control of brightness of the illumination field but there is also a small change in color due to a shift in the temperature of the filament. Long-lifetime halogen bulbs require maintenance of a narrow current range for proper operation to ensure the redeposit of metals (e.g., tungsten) by the halogen gas. The operating current of a long-lifetime bulb cannot be varied for significant control of brightness at the illumination field. Other methods for brightness control affect the spatial and angular uniformity of the illumination field. Poor coupling efficiency and inadequate brightness control frequently eliminate a long-lifetime bulb from use in a microscope.
Another illumination technique involves a light emitting diode (LED), which is bright but also has a small active area with large errors in placement and fixation (i.e., alignment). An LED can easily accommodate the poor coupling efficiency of diffusion within fixed elements, but the spatial uniformity can display a deviation of 30 to 40% from its peak. Further, the resulting illumination may be non-uniform in angle. LEDs display a wide variety of angular profiles, none of which are uniform. LEDs do not display smooth wavelength spectrums. Consequently, the color of an LED system cannot duplicate the blackbody spectrum of an incandescent lamp. The color spectrum of an LED is suitable for a machine vision system. However, it is not typically acceptable to a human observer looking through a microscope. Consistent color temperature cannot be achieved by an LED. LEDs have long lifetimes of 10,000 hours or more.
Thus, what is needed is a solution for isotropic illumination with high coupling efficiency and ease of installation by an operator to permit the use and maintenance of a long-life incandescent bulb. Further, a solution where variation of brightness may occur without an associated change in color temperature is also desirable.